[Solid Electrolytic Capacitor]
Late years, operating frequencies of electronic devices have become higher and higher. Along with this trend, an electrolytic capacitor as one electronic component also needs to be provided as a product having excellent impedance properties in a higher operating frequency range than before. In order to cope with the need, various solid electrolytic capacitors using, as a solid electrolyte, an electrically conductive polymer with a high electrical conductivity, have been developed. This type of solid electrolytic capacitor is excellent, particularly, in high-frequency properties, in addition to life and temperature properties, and thereby widely employed in electric circuits for personal computers, and others.
In one simplest example, a wound-type solid electrolytic capacitor can be produced by a process comprising a step (i) of: subjecting a surface of an anode aluminum foil to a chemical conversion treatment to form an oxide film thereon; laminating the resulting anode aluminum foil to a cathode aluminum foil through a separator sheet; connecting a lead member to each of the two foils; and winding the laminate to prepare a capacitor element, and a step (ii) of: placing the prepared capacitor element in an aluminum casing; immersing the capacitor element in an electrically conductive polymer solution to cause thermal polymerization of an electrically conductive polymer under heating to form a solid electrically conductive polymer layer between the two foils. When the anode aluminum foil is used as an anode, and the cathode aluminum foil and the electrically conductive polymer layer electrically connected to the cathode aluminum foil are used as a cathode, the anode and the cathode are connected through the electrically insulating oxide film, so that it becomes possible to achieve charge and discharge between the anode and cathode.
In the above solid electrolytic capacitor, the cathode aluminum foil is not subjected to a chemical conversion treatment, so that no artificially formed oxide film exists thereon. However, actually, an oxide film is also formed on the cathode aluminum foil, due to natural oxidation during production or use. In this case, the solid electrolytic capacitor is generally formed in a layered structure comprising (i) the anode aluminum foil, (ii) the oxide film on the anode aluminum foil, (iii) the electrically conductive layer, (iv) the natural oxide film on the cathode aluminum foil, and (v) the cathode aluminum foil. This is equivalent to a state in which two capacitors are connected in series to each other, which causes a problem that a capacitance of the solid electrolytic capacitor as a whole is reduced.
In order to cope with this problem, researches have been conducted to prevent a capacitance component from being generated in a cathode to thereby allow an increase in capacitance of a capacitor. In this connection, some cathode foils obtained from the conventional researches and a problem involved in the conventional cathode foils will be described later.